The portrait was painted just after
Newton published
Philosophiae
Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy") (1687)
which presented
Newtonian physics
and the first physically-correct description of
Solar System
(in the
Book III:
De mundi systemate (On the System of the World):
Newton
was the greatest genius that ever existed,
and the most fortunate,
for we CANNOT find more than once a system of the world to establish.
---Joseph-Louis Lagrange (1736--1813)
quoted from Joseph-Louis Lagrange Quotes.